The Synchronous Optical Network (SONET) was created as the standard fiber optic transmission system for long-distance telephone and data communications. SONET is most commonly described as a high bit-rate fiber-optic-based transport method that provides the foundation for linking high-speed ATM (Asynchronous Transfer Mode) switches and multiplexers and providing users with B-ISDN (Broadband-Integrated Services Digital Network) compliant services. SONET has a very detailed architecture and also contains a series of protocols to implement this architecture. As the architecture is quite complex, this document will only describe those portions of the protocols and frame structure that are useful for explanation purposes.
First, the SONET hierarchy consists of a number of levels, organized according to data transmission speed. Each level has an optical carrier (OC) and an electrical level transmission frame structure, termed the synchronous transport signal (STS). The notation OC-N refers to the nth level of the optical carrier. The counting units are the basic 51.84 Mbps bit rate of OC-1. OC-3, for example, has a bit rate of 155.52 Mbps which is derived from the 3 times multiplier of OC-1. The STS-N notation follows that of the OC-N. For example, an STS-3 frame is sent on an OC-3 fiber link at 155.52 Mbps. The payload, a term commonly referred to, is used to indicate user data within a SONET frame.
The American National Standards Institute (ANSI) T1.105-1995 standard, titled “Synchronous Optical Network (SONET)—Basic Description including Multiplex Structure, Rates and Formats,” further elaborates on the SONET frame formats and the data rates previously mentioned.
The basic unit of SONET transmission capacity (51.84 Mb/s) is encompassed by a frame structure consisting of 90 columns and 9 rows (810 bytes), and designated as an STS-1 frame, with each byte in this frame representing a timeslot. The capacity of a SONET optical link is increased by byte-interleaving or multiplexing several basic STS-1 frames to generate larger and larger total information carrying capacity. However, each individual STS-1 frame remains separate from the others within this multiplexing structure and is managed as a separate unit carrying a different information stream from all others. This is satisfactory when the individual streams of information to be transported are less than 51.84 Mb/s in capacity.
In many situations it may be necessary to transport more than 51.84 Mb/s of data information between two end-points as a single stream. SONET networks employ a method called concatenation to increase the capacity allocated to any given data stream or path, but still transport and manage the path as a single unit. Concatenation refers to the process whereby multiple basic STS-1 frames are linked together to create a larger frame that is then transmitted and managed as a unit. Until recently, the only form of concatenation supported was contiguous concatenation, wherein the number of basic frames permitted to be so linked are certain fixed multiples of 3 (3, 12, 48, 192, 768) and the linked frames are required to have a fixed relationship in time to each other when transported over a SONET network. Contiguously concatenated SONET frames are designated as STS-3c, STS-12c, STS-48c, STS-192c and STS-768c, respectively, depending on the number of STS-1 frames that are linked to generate the concatenated frame.
While the restrictions placed upon contiguously concatenated frames greatly simplify the implementation of equipment, it unfortunately also unduly constrains the flexibility afforded to a user in creating and configuring a SONET network, and also impacts the efficiency of the data transfer process. For instance, a popular requirement within a SONET network is to be able to transport Fast Ethernet or Gigabit Ethernet signals. These signals have fixed data rates of 100 Mb/s and 1000 Mb/s respectively. The smallest contiguously concatenated SONET frames that are capable of carrying these signals are STS-3c (155.52 Mb/s) and STS-48c (2488.32 Mb/s), respectively. However, the corresponding resulting transmission efficiencies are only 64% and 40%. This is because the allocated bandwidth of the SONET contiguously concatenated frame greatly exceeds the required bandwidth by the Ethernet signals. In addition, the number of different signals that may be combined together into one link is greatly restricted by this waste of bandwidth due to inflexibility.
A standard for transmission of information over SONET networks calls for the use of virtual concatenation to solve this problem. Virtual concatenation attempts to link together multiple 810-byte STS-1 sub-frames in the same manner as contiguous concatenation, but eliminates all of the restrictions on the number of basic STS-1 sub-frames thus linked as well as the relative placement of these STS-1 sub-frames within the complete SONET frame structure. In addition, the STS-1 sub-frames constituting a virtual concatenation frame are only associated with each other at the end-points; the intervening SONET network is free to route these subframes by different paths if this results in simpler network configuration or better network utilization. The result is that provisioning flexibility and transmission efficiency are greatly increased. In the Fast Ethernet and Gigabit Ethernet examples provided above, the use of virtual concatenation allows a user to link 2 STS-1 sub-frames (103.68 Mb/s) and 20 STS-1 subframes (1036.8 Mb/s), respectively, to carry the Ethernet signals. The resulting efficiencies are 96.5% for both cases, which is a great improvement over the contiguous concatenation case.
Virtual concatenation also permits the linking of several contiguously concatenated frames in place of the basic STS-1 frames if so desired. For instance, an alternative to linking STS-1 sub-frames is to link STS-3c sub-frames to obtain the necessary bandwidth. This is done if a reduction in management and complexity is required, as the number of STS-3c sub-frames that must be linked to yield a particular bandwidth is one-third the number of STS-1 sub-frames for the same equivalent bandwidth. Processing such virtual concatenation frame streams, however, presents a significant challenge for a SONET equipment designer, especially as the data rates increase to 2.5 Gb/s (oC-48) and more. As each STS-1 sub-frame may take a different path from source to destination, a differential delay may exist between the sub-frames constituting a single virtual concatenation frame, and this differential delay must be eliminated by time-shifting the sub-frames arriving at a destination before recombining them. Also, the arbitrary positioning and number of sub-frames comprising a virtual concatenation frame means that complex shifting and combining must be done (after compensating for differential delay, as described). Virtual concatenation also supports means whereby the number of sub-frames assigned to a channel may vary dynamically to account for changing conditions, which renders the construction of sub-frame interleaving and de-interleaving apparatus very difficult. Finally, virtual concatenation does not adhere to the convenient multiples of 4 utilized by contiguous concatenation, resulting in much more complex multiplexing and demultiplexing arrangements.
The presence of an arbitrary, and dynamically varying, number of virtual concatenation frames within a SONET data stream makes the traditional approach to equipment design, which is to use dedicated separate processing units for the different types of concatenated frames, infeasible. A better approach uses some form of time-sliced arrangement, whereby a single SONET payload processor would be combined with some kind of dynamic context switching capability to create the effect of an arbitrary number of different payload processors. A significant issue with the time-sliced approach, however, is that it is difficult to adapt to the problem of handling arbitrary multiplexing ratios. For example, processing a random combination of STS-1 sub-frames with some arbitrary grouping into virtual concatenation streams is difficult to manage, due to the complexity of switching context information at high rates. In addition, the handling of higher-speed SONET data rates (2.488 Gb/s and above) forces the datapaths used to process these frames to be made more than one byte wide, and wide time-sliced datapaths are notoriously difficult to design and configure.
The problem of handling higher-speed SONET data rates has not been commonly encountered to date for two reasons. Firstly, the faster SONET rates have only recently begun to be used, and the problem becomes significant only when these higher rates need to be processed. Secondly, the virtual concatenation procedure is itself very new; the formal description for SONET virtual concatenation, as embodied in American National Standard Institute (ANSI) T1X1.5/2001-062 standard, was published in 2001. The construction of a SONET payload processor suitable for virtual concatenation processing, therefore, is a new area in which little activity has occurred to date.
The prior art has not provided a solution to the virtual concatenation problem. U.S. Pat. No. 6,160,819, issued to Partridge et. al., discloses a form of inverse multiplexing of variable-length packet data over multiple low-speed SONET links that bears some general resemblance to the virtual concatenation technique, but presupposes the existence of a Byte-By-Byte (BBB) “striping unit” that performs the actual splitting and recombining of the data stream, without giving details about its construction. U.S. Pat. No. 6,147,968, issued to De Moer et. al., discloses an apparatus that is capable of splitting a SONET ring into blocks and sub-blocks, but limits itself to the control issues of delineating various sub-blocks within the complete ring using a marking scheme, without any details as to how the component blocks and sub-blocks (representing concatenated payloads) can be constructed and disassembled in an efficient and physically realizable manner.
A co-pending U.S. patent application Ser. No. 10/176,230 filed Jun. 3, 2002, entitled “Efficient Variably-Channelized SONET Multiplexer and Payload Mapper” describes a variably-channelized SONET mapper arrangement that is substantially more efficient at processing contiguously concatenated streams than the prior art at high speeds, but this is also inapplicable to the task of processing virtual concatenated frames due to its multiplexing and demultiplexing in powers of 2 rather than arbitrary combinations.
In view of the above shortcomings, the present invention provides a SONET payload processing system capable of handling virtual concatenation, as well as contiguous concatenation in an efficient and straightforward manner.